1. Field of the Invention
The present invention is directed to an electrically driveable shockwave source of the type for generating acoustic shockwaves suitable for medical therapy, and in particular to such a shockwave source having a coil arrangement with a membrane disposed opposite the coil arrangement.
2. Description of the Prior Art
Electrodynamic shockwave sources are used, for example, for medical therapy for the non-invasive disintegration of calculi, for treating pathological tissue conditions, or for the treatment of bone diseases. Such shockwave sources are operated by charging the coil arrangement with a high-voltage pulse. The membrane consists of electrically conductive material, and as a result of the charging of the coil arrangement, currents are induced in the membrane in a direction opposite to the direction of the current flowing in the coil arrangement. As a consequence of the opposing magnetic fields arising due to the opposite flows of respective currents in the coil arrangement and the membrane, repulsion forces are exerted on the membrane causing the membrane to suddenly and rapidly move away from the coil. A pressure pulse is thereby introduced into an acoustic propagation medium adjacent the membrane. The pressure pulse intensifies into a shockwave as it propagates through the medium as a consequence of the non-linear compression properties thereof. In the present discussion, however, the term "shockwave" will always be used, and will encompass within its meaning an incipient shockwave, i.e., a pressure pulse.
If necessary, the shockwave may be concentrated onto a focus zone with suitable focusing means, for example, an acoustic lens, or by appropriate shaping of the shockwave source, for example, fashioning the membrane and the coil in the form of a portion of a sphere.
The shockwave source and the subject to be acoustically irradiated are acoustically coupled to each other in a suitable manner, and are aligned relative to each other so that the region to be acoustically irradiated is situated in the focus zone of the shockwave source.
In order to achieve an optimum conversion of the electrical energy supplied to the shockwave source into acoustic shock energy, it is necessary to attach the membrane as closely as possible to the coil arrangement. In conventional shockwave sources, however, the closeness of the membrane to the coil arrangement is limited due to the difference in potential which exists between the coil arrangement and the membrane. A minimum spacing must be observed in order to avoid voltage arcing. Voltage arcing would deteriorate the operation of the shockwave source, and would lead to damage of the membrane and thus to premature failure thereof. In order to insure an adequate service life of the membrane, the necessary distance which must be maintained between the membrane and the coil arrangement results in an extremely low efficiency in the conversion of electrical energy into acoustic shock energy. Apart from the fact that this is an unsatisfactory circumstance insofar as efficiency, a further disadvantage is that relatively complicated measures for must be undertaken for eliminating the considerable heat which is dissipated due to the low efficiency of the shockwave generation.
The problems of low efficiency and high heat generation are most acute in shockwave sources of the type described above wherein the membrane consists of metal, as described, for example, in U.S. Pat. No. 4,674,505. One proposed solution to these problems is disclosed in European Application 0 266 538, corresponding to U.S. Pat. No. 4,793,329, wherein the membrane consists of an insulator disc on which electrically conductive sections are arranged in the form of concentric rings. This structure creates an insulation path having an extremely long length, which must be overcome before voltage arcing can occur, thereby permitting the membrane to be arranged relatively close to the coil arrangement.
Another electrodynamic shockwave source is disclosed in European Application 0 256 232, corresponding to U.S. Pat. No. 4,796,608, wherein the coil consists of two parallel, superimposed, series-connected layers with one of the layers having a smaller difference in potential with respect to the membrane, and that layer being disposed directly opposite the membrane. Because the voltage at the coil arrangement drops across the coil, a difference in potential is present between the membrane and the layer immediately adjacent thereto, which is lower than the magnitude of the voltage at the coil. The membrane can thus be situated in relatively close proximity to the coil.
Although the risk of voltage arcing is minimized in these known structures, the efficiency is still not entirely satisfactory.